The present invention relates to an automobile-mount diesel engine with a turbocharger, and particularly relates to a control of a fuel injection in the diesel engine.
In automobile-mount diesel engines, a fuel injection is performed in each of cylinders for more than once during one cycle of the engine in order to, for example, reduce NOx and soot contained in exhaust gas, reduce noises or vibrations, and improve a fuel consumption and torque. For example, JP2009-293383A discloses a diesel engine with a turbocharger that performs fuel injections at five timings: a main injection for generating a torque, a pilot injection performed prior to the main injection so as to preheat cylinders, a pre-injection performed between the pilot injection and the main injection so as to suppress an ignition delay of fuel injected by the main injection, an after injection performed after the main injection so as to raise a temperature of exhaust gas, and a post injection for raising a temperature of a catalyst by directly introducing the fuel to an exhaust system subsequent to the after injection.
Meanwhile, in the diesel engine, an EGR (Exhaust Gas Recirculation) for re-circulating combusted gas is performed particularly within a partial load range within a low engine speed range (here, the partial load range is defined as a range with a middle load when an operating range of the engine is partitioned into three ranges, high, middle and low loads, respectively, and the low engine speed range is defined when an operating range of the engine is partitioned into two ranges, the low engine speed range and a high engine speed range) so that a discharge amount of NOx is reduced. However, within an operating range where a large amount of EGR gas is introduced, an amount of air inside cylinders decreases, and therefore, it is disadvantageous in reducing the soot discharge. In such case, performing pre-stage injection twice, such as the pilot injection and the pre-injection, prior to the main injection, as disclosed in JP2009-293383A, increases a premixture ratio of fuel and is beneficial in reducing the soot.
Within such an operating range of the engine where the load is higher than within the operating range where the large amount of the EGR gas is introduced, for a turbo charged engine, a turbocharging amount increases and an injection amount of the fuel increases as a result of the increased load, and thus, an ignitability within the cylinders is improved. Therefore, within the relatively high load range, where the ignitability is improved, performing the pre-stage injection twice increases a heat release rate (dQ/dθ) in a pre-combustion before a main combustion caused by the main injection to be comparatively higher. Thus, two high peaks of the heat release rates are generated in the pre-combustion and the main combustion, and thereby, combustion noises with low frequency in the combustions overlap with each other and become louder and cause a problem of degradation in an NVH (Noise, Vibration and Harshness) performance. Particularly, because the partial load range within the low engine speed range is comparatively frequently utilized, and the degradation of the NVH performance easily becomes a problem.